The present invention relates to dynamic measurement of load on a sucker rod pump and, more particularly, to a method and apparatus for measuring the load on the sucker rod.
The most common means of artificial lift in oil wells is sucker rod pumping. The basic elements of a sucker rod pumping system are illustrated in FIG. 1. These systems include a prime mover 2, typically an electric motor, which drives a gear box and counterweight unit 4. Gear box 4 in turn drives one end of walking beam 6. The opposite end of walking beam 6 is connected to the upper end of a sucker rod string 8 via two cables connected to the walking beam. These cables are often referred to as the bridle. Sucker rod string 8 extends from the earth's surface to the actual pump 10 located at the bottom of cased well 12.
The major expenses involved in operation of a sucker rod pumping system are the electrical power input needed for motor 2 and expenses of repairing pumps that fail. It is desirable to pump at the highest rate possible to maximize oil production without allowing the pump to go dry which can cause mechanical failure of the pump 10. Various adjustments can be made to the motor speed, counterweight and stroke length to optimize production while minimizing energy use and breakdowns.
The operation of the sucker rod pumping system is optimized using a dynamometer, a device which records load versus displacement during the pumping cycle. Electronic dynamometers are becoming more common. Electronic dynamometers, especially when coupled with computers, greatly speed analysis. On high production wells, dedicated electronic dynamometers, or pump-off controllers, are sometimes used to continuously monitor pumping unit operation, thus assuring maximum performance and minimizing failures.
The primary input to a dynamometer or pump-off controller is the time varying load on the sucker rod string, and in particular, to the polished rod portion positioned at the upper end of the string. Pump-off controllers typically use a permanent electronic load cell attached to the polished rod to measure rod load. These load cells are expensive and difficult to install and remove.
Clamp-on load cells have also been developed for use with portable electronic dynamometers. These load cells simply collar around the polished rod and are quickly and easily installed or removed. These devices are generally known as extensiometers, measuring the strain of the polished rod with varying load. Because they are normally installed onto the polished rod with the rod already carrying a load, they do not measure the strain resulting from pre-existing load. Thus, the collar-on load cells measure only relative load changes, not absolute loads. In order to measure absolute loads with the extensiometer-type load cell, the load, typically 5,000-10,000 pounds, must be taken off the polished rod prior to installation.